One of the most important aspects in developing lubricant compositions for various applications is the individual adjustment and optimization of friction properties. Varying applications have differing demands on a lubricant. For example, a gear oil will require good extreme pressure and load-bearing properties, generally low boundary friction and low thin film friction is desired. On the other hand, a lubricant optimized for continuously variable transmissions (CVT) will require low boundary friction and high thin film friction. Clutches, in particular in automatic transmissions, require high boundary friction for an optimal torque transfer, whereas low thin film friction is generally desired to efficiently pump fluid through the transmission.
A particular focus of automatic transmission fluids (ATF) is the minimization of shudder (anti-shudder properties), which is believed to be a function of the change of friction coefficient with time (dμ/dt<0). Furthermore, shift characteristics of automatic transmissions are primarily dependent on the frictional characteristics of the ATF. The fluid needs to have a very stable frictional performance over the life of the fluid. Generally, it is desired to design ATF lubricant compositions that maximize service intervals or even better, avoid oil service during the lifetime of the equipment. This is referred to in the industry as a lifetime fill or “fill-for-life” fluid. Therefore, maintenance of the friction properties of an ATF over time, i.e. ATF friction durability, is a highly desired property.
This is especially true in the field of power transmissions, in particular automatic transmissions, where an increasing demand for lubricant compositions with such improved friction properties exists. Accordingly, several approaches for providing a lubricating composition with improved friction performance are known. One approach is using high levels of amines as friction modifiers to improve the friction properties of a lubricant. The use of such amines at high levels has two problems. First the use of these amines will, because they are typically such effective friction reducers, drop the static friction performance of the ATF in a transmission to unacceptable levels causing the transmission to not be able to transmit torque effectively. Second, the use of high levels of amines often has a deleterious effect on the various polymeric seal materials, e.g., the seal material classed as GM DEXRON® IIIH Elastomers, e.g., V3 (VAMAC), P2 (polyacrylate), N1 (nitrile), F1 (fluoroelastomer), etc., used in transmission hardware.
In EP 0 721 978 B1 a power transmission fluid composition comprising a hydrogenated poly-alpha-olefin oligomer fluid, an acrylic viscosity index improver, a dialkylester, an ashless dispersant, a friction modifier and an inhibitor is disclosed. Furthermore, EP 0 721 978 B1 suggests the use of amine phosphate as anti-wear extreme pressure agent in an amount of 0.05 wt.-%.
EP 0 713 908 B1 describes power transmission fluids having a Brookfield viscosity of 13,000 mPa s or less at −40° C. comprising at least 50 wt.-% of 55 N to 125 N hydro treated mineral oils, 5 to 40 wt.-% of hydrogenated poly-alpha-olefin oligomer, 5 to 20 wt.-% of acrylic viscosity index improver and other additives components. The compositions possess a combination of performance properties necessary for electronically controlled automatic transmissions equipped with torque converter clutches capable of operating in a continuous slip mode. Furthermore, EP 0 713 908 B1 suggests the use of amine phosphate as anti-wear extreme pressure agent in an amount of 0.02 to 0.05 wt.-%.
Again, two important friction properties of ATF compositions are friction durability and minimum static friction. EP 0 856 042 B9 describes two conventional ways to improve friction durability. One way is to increase the amount of friction modifier in the fluid which improves the friction durability, but, as discussed above, adversely affects, i.e. lowers, the friction coefficient of the fluid to undesirable levels, especially the static coefficient of friction. An alternative method is to improve the oxidation resistance of the fluid because it is generally believed that polar oxidation products compete with the friction modifiers for the friction surface. Nevertheless, reducing fluid oxidation for improving long term control of friction is difficult and hardly predictable. EP 0 856 042 B9 suggests to use a composition comprising a mixture of an antioxidant, a low potency friction modifier and an oil-soluble phosphorus containing compound, for improving the friction durability of an automatic transmission fluid.